1. Field of the Invention
The present invention relates to a pulse generating capacitor, for example, a pulse generating capacitor that is incorporated into a high intensity discharge (HID) lamp.
2. Description of the Related Art
In pulse generating capacitors that are used for generating high-voltage starting pulses for HID lamps, electrodes and lead terminals of the capacitors are conventionally bonded together with bonding members composed of Ag-containing a low-melting-point glass material. When the glass content of the bonding members is excessively high, the glass component diffuses into the grain boundaries of the capacitor element (ceramic board) during baking, resulting in a decrease in the pulse voltage of the capacitor. Furthermore, when the capacitor is used at a high temperature, the diffusion advances with the passage of time, resulting in a further lowering of the pulse voltage, as well as a lowering of the impact resistance of the capacitor element. On the other hand, when the glass component content is too low, the strength of the bonding members is essentially insufficient, causing problems such as lowered bonding strength.
As a countermeasure, Japanese Unexamined Patent Application Publication No. 2-177412 teaches that a bonding member having little effect on the pulse voltage and having an adequate strength can be selected by limiting the content of a glass component in the bonding member to the range of 39% to 69% by weight. Furthermore, Japanese Unexamined Patent Application Publication No. 63-136612 discloses an insulating layer provided between the bonding members and the electrodes for preventing diffusion. However, even when such measures are adopted, lowering of the pulse voltage will occur to some extent as long as a glass material is employed.
Accordingly, Japanese Unexamined Patent Application Publication No. 11-274000 proposes the use of an alloy which does not include a glass material for the bonding member, so as to prevent the pulse voltage from decreasing due to diffusion of the glass material and to enhance the bonding strength. However, it was found that the use of such a bonding member caused cracks on a capacitor element due to shrinkage-caused stresses of the bonding member in a thermal cycle test, resulting in breakage of the capacitor element.
Based on considerations on the problems described above, one of the objects of the present invention is to provide a pulse generating capacitor that can alleviate the problem of crack generation on the capacitor element as a result of temperature changes, as encountered for instance during a thermal cycle test, while maintaining the effects of preventing the pulse voltage from decreasing and enhancing the bonding strength of the lead terminals.
To achieve the above-described object, a pulse generating capacitor according to the present invention comprises a plate capacitor element, electrodes both on the front and rear faces of the capacitor element, and lead terminals connected to the electrodes by means of bonding members, wherein the bonding members comprise a metal as a major component with a ceramic powder added thereto. The ceramic powder is preferably zirconium oxide. It is also preferable that the content of the ceramic powder is in the range of about 20% to 60% by volume.
In the pulse generating capacitor according to the present invention, the bonding members comprising metal as a major component with a ceramic powder added thereto are used. Accordingly, the linear thermal expansion coefficient of the bonding members is close to that of a capacitor element comprising a ceramic as a major component, resulting in a smaller thermal stress in a thermal cycle test and a smaller thermal conductivity of the bonding members, as well as a smaller temperature distribution of the product (capacitor). Furthermore, since no glass material is added or only a small amount of a glass material is added, there is no decrease in the pulse voltage or in the bonding strength.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.